The present invention relates to a process for producing 4,4'-dihydroxydiphenyl and, more particularly, a process for obtaining high purity 4,4'-dihydroxydiphenyl in a high yield by the alkali fusion of 4,4'-diphenyldisulfonic acid with potassium hydroxide.
Recently, the range of use of heat-resistant synthetic polyester resins as engineering plastics has been increasing. For realizing functional polymer characteristics of the engineering plastics, it is indispensable requisites that high purity monomers be used as the starting material and contamination of the polymer with unreacted matters, isomers or inorganic matters be avoided as far as possible.
4,4'-dihydroxydiphenyl (hereinafter referred to as DHDP) is an important monomer used as a starting material of the engineering plastics.
Generally, DHDP is produced by the alkali fusion of 4,4'-diphenyldisulfonic acid (hereinafter referred to as DPDS). For facilitating the stirring of the reaction system to accelerate the hydroxylation reaction, the following processes have been employed in general: (1) a process wherein a large excess amount of sodium hydroxide is used, (2) a process wherein benzenesulfonic acid is used as a solvent in the alkali fusion reaction, (3) a process wherein the reaction temperature is lowered and potassium hydroxide is used for inhibiting the side reactions to increase the yield, (4) a process wherein potassium hydroxide and sodium hydroxide are used together and (5) a process wherein the use of a large excess amount of an alkali is avoided.
As process (1), there may be mentioned, for example, a process wherein 14-40 mol of sodium hydroxide is used (Japanese Patent Laid-Open No. 112844/1979). In this process, however, complicated steps are required for recovering excessive caustic soda and concentrating the same after completion of the reaction and yet the sodium hydroxide recovery is poor. This process is, therefore, economically disadvantageous.
Namely, after completion of the reaction, the reaction mixture comprises an excessive alkali containing the intended alkali metal salt of DHDP and inorganic salts such as an alkali metal sulfite formed in a rate of nearly 2 mols per mol of the disulfonic acid.
Thus, process (1) inevitably has problems of (a) the recovery of the excessive alkali in a reusable state and (b) the removal of inorganic salts which contaminate intended DHDP. Some processes for solving these problems have been proposed.
For example, for solving problem (a), there has been proposed a process wherein only the excessive alkali is recovered with water and reused (Japanese Patent Laid-Open No. 112844/1979). However, in this process, it is difficult to separate inorganic salts from the alkali metal salt of DHDP without causing a loss of the latter.
For solving problem (b), there has been proposed, for example, a process wherein the alkali metal salt of DHDP is extracted with an alcohol having up to 5 carbon atoms (Japanese Patent Laid-Open No. 108031/1974). This process has a defect that the alkali cannot be recovered or reused, since the excessive alkali is also extracted.
As process (2), there may be mentioned a process of U.S. Pat. No. 2,368,361 which has an advantage that phenol is also obtained by the alkali fusion of benzenesulfonic acid used as the solvent. However, this process is now unattractive, since a more advantageous process for producing phenol alone has been established already.
In processes (3) and (4), an alkali is used generally in a large excess amount in order to facilitate the stirring by reducing the viscosity of the reaction mixture or because of the restriction in the procedure to be effected after completion of the reaction.
Therefore, the processes (3) and (4) have disadvantages that the alkali recovery rate is poor and the effective separation of inorganic salts from DHDP is difficult as in process (1).
As for process (5), there may be mentioned, for example, a process of Japanese Patent Laid-Open No. 52142/1976 which comprises adding sodium hydroxide flakes to powdery sodium 4,4'-diphenyldisulfonate while the reaction system is kept always in a powdery state. In this process, it is difficult from the viewpoint of the operation to keep the reaction system always in a powdery state, since water is formed by the reaction. Another disadvantage of this process is its poor yield.